This invention relates to the insulation of deepcooled cables such as super conducting three-phase cables in a coaxial conductor configuration in general, and more particularly to insulation of such cables using insulating foils having a dielectric loss factor smaller than 5 .times. 10.sup.-.sup.5.
In previously disclosed conductor systems for use in super conducting three-phase cables it is essential that all current losses be avoided in those portions of the cable arranged in the region of the evaporation temperature of liquid helium of about 4K. Thus, arrangements of such cables in which the electromagnetic field acts only between the superconductors are preferred. Because of this, each of the phase conductors of tubular shape has associated with it a coaxial return cable with the three phases of the three-phase cable interlinked at the cable end. As a result, the three return conductors are essentially at the same potential. In such coaxial arrangements, for example, one or plurality of layers of individual, stabilized superconducting wires are employed as conductors. In addition, the layers of superconducting wires can be twisted in such a manner to insure a cable length independent of the temperature when cooled down to low temperature. The concentric arrangement of the superconducting layers with respect to each other is made possible through the use of an electrical insulation layer of tubular design between the respective layers.
The higher limit of current capacity in a superconducting cable made of a superconductive material such as niobium is limited by the critical surface flux density. Because of this, the dielectric properties of the insulating layer of a super-conducting three-phase cable is of great importance for its transmission capacity, and, therefore, for its economy. With the cable diameter generally being fixed, the conductor current cannot exceed a given value without the danger of a transition to the normally conducting state in the superconductive material because of its critical surface flux density limitations. As a result in order to transmit large amounts of electrical power a superconducting three-phase cable must be designed for the highest possible transmission voltages thereby requiring extremely excellent dielectric properties in the electrical insulating layer.
In addition to the high dielectric strength at low temperatures the insulation of the superconducting three-phase cable must also have a very small dielectric loss factor tangent .delta. where tangent .delta. equals EQU tan .delta. = V.sub.d / U.sup.2 . .omega. . .epsilon. C.sub.o
where V.sub.d represents the dielectric losses, U the operating voltage, .omega. the singular frequency and .epsilon. C.sub.o the capacity of the cable. A very small factor, tangent .delta., of approximately 5 .times. 10.sup.-.sup.5 which is extremely difficult to achieve using known cable insulating materials, such as those used in connection with high voltage oil cables is necessary since the dielectric losses V.sub.d must be removed in a superconducting cable by the refrigeration equipment which operates at low efficiency. For example, it takes 400 W of refrigeration to remove to the atmosphere losses on the order of 1 W occurring at the helium temperature. The use of paper insulation having a loss factor tangent .delta. of several times 10.sup.-.sup.3 such as is used in oil cable technology is therefore out of the question for use in superconducting cables.
"Elektrotechnische Zeitschrift," Series A, Vol. 92 (1971), pages 740 to 745, teaches the use of helium impregnated foil insulation, in which for example, polyethylene, polytetrafluorethylene or polyfluorethylene propylene is used as the foil material to form insulation between concentric conductor layers. Insulation comprising individual layers of foil which may also be saturated with the coolant of the cable is more advantageous for deep-cooled cables than insulation of solid plastic material which has a thermal contraction differing greatly from that of the metallic cable conductors. The cooling liquid saturated foil insulations often has a higher dielectric strength than the cooling liquids themselves and have relatively low dielectric losses. According to data in "Report BNL 50 325," Brookhaven National Laboratories, March, 1972, dielectric loss factors are approximately 2 .times. 10.sup.-.sup.5. However, the dielectric constant .epsilon. of these plastics is relatively high. Polyethylene, for example, has an .epsilon. of about 2.25. This in turn can cause higher dielectric losses.
A further principal difficulty in operating a deepcooled cable is the different shrinkage of the materials used. The shrinkage of the metal of the conductors is about 2 to 4 parts per thousand whereas the above mentioned plastics have a shrinkage of up to several percent. As a result, a strong axial pull will be exerted on rigid plastic insulation arranged between the conductor layers when the cable is cooled down. However, since the adhesion forces of two adjacent foils are large in the case of polyethylene, the danger of a foil package wound with such foil being town apart in one or a number of places due to the large forces acting on it exists.
In German Offenlegungsschrift No. 1,490,320, layered insulation for high voltage purposes consisting of layers of plastic foil with interposed layers of tape fabric permeable for a liquid impregnant, e.g., oil, and whose dielectric constant is approximately equal to that of the plastic foil is disclosed. The disclosed purpose of the insulation arrangement therein, however, is to insure proper impregnation of the insulation and to make possible a voltage gradient within the insulation which is as uniform as possible.
In view of these various difficiencies in prior art insulation for superconducting cables, it is the object of the present invention to create plastic insulation having sufficient mechanical elasticity for operation at cryogenic temperatures without appreciable degradation of its dielectric loss factor when compared to solid plastic insulation.